Abstract
Purpose
The objective of this study was to investigate the end-tidal carbon dioxide concentration (ET co 2 ) monitoring in obstructive sleep apnea (OSA) patients during sleep and to explore whether the ET co 2 value may explain a significant portion of the relationship between ET co 2 value and apnea/hypopnea index (AHI) and nocturnal oxygenation indices.
Materials and methods
Thirty-eight consecutive patients underwent overnight polysomnography and were synchronously monitored for ET co 2 using an microstream capnometer. Mean and maximum values during wake time and different sleep stages were recorded.
We grouped 38 OSA patients into 2 subgroups on the basis of their difference of mean total sleep time and wake time ET co 2 [(T − W) ET co 2 ]; one group, 20 patients with (T − W) ET co 2 less than 0, and the other group,18 patients with (T − W) ET co 2 greater than 0.
Results
Group with (T − W) ET co 2 less than 0 patients exhibited higher AHI (mean ± SD, 68.58 ± 22.78 vs. 27.61 ± 19.44 events/h) and lower nocturnal oxygenation indices (minimum Sa o 2 , 67.85 ± 10.08 vs. 82.61% ± 6.07%; mean Sa o 2 , 91.29 ± 3.31 vs. 95.15% ± 1.88%) compared with the other group.
Conclusions
In summary, the study provides preliminary data showing that ET co 2 potentially can be used in continuous monitoring of OSA patients. And, (T − W) ET co 2 can indicate the severity of OSA.
1
Introduction
Capnography is the noninvasive measurement of the partial pressure of carbon dioxide (CO 2 ) in exhaled breath displayed as a numerical value and a waveform. The CO 2 waveform, or capnogram, displays changes in the CO 2 concentration over the respiratory cycle. Trends in ET co 2 values can be used to assess disease severity and response to treatment, whereas changes in the shape of the capnogram are diagnostic of many disease conditions .
Capnography is now extensively used in emergency medicine to monitor endortracheal tube position and verify endotracheal tube placement, monitor ventilatory status of respiratory-impaired patients, monitor ventilation of patients during sedation and analgesia, evaluate ventilator settings and circuit integrity, assess effectiveness of cardiopulmonary resuscitation, and early detection of changes in airway resistance and circulatory collapse .
Although ET co 2 measurement or capnography has broad applications in the emergency department, in preoperative and intensive care, it is also to be used to assess ventilatory status in spontaneously breathing patients and for the evaluation of sleep-related breathing disturbances in recent years . It is noninvasive and easy to apply to breathing circuits. The ET co 2 measurement and capnography has been studied for sleep apnea syndrome to detect hypopnea and apnea .
Obstructive sleep apnea (OSA) is a sleep-related and respiratory disorder characterized by recurrent airflow obstruction caused by total or partial collapse of the upper airway . The “gold standard” for a definitive diagnosis of OSA is polysomnography (PSG) . The apnea/hypopnea index (AHI) is a measure of the frequency of respiratory events and is generally considered to be statistically significant in adults when 5 or greater.
Several studies compared the AHI calculated from the ET co 2 waves and the corresponding polysomnographic data and demonstrated that a capnography-based AHI significantly correlates with the AHI as measured by traditional PSG . However, few studies have been reported on continuous monitoring and analyzing overnight ET co 2 values in patients with OSA during sleep.
The objectives of this study were to (1) record ET co 2 value via microstream capnometer using an oral-nasal cannula in patients with OSA and (2) explore the relationship between the ET co 2 values with the parameters from PSG.
2
Materials and methods
2.1
Patients
The protocol was approved by the Institutional Review Boards of Beijing Tongren Hospital (China). All participants gave informed written consent. Thirty-eight patients who were referred for suspected OSA due to the presence of symptoms suggestive of OSA, that is, habitual snoring, daytime sleepiness, and witnessed apnea, were conducted to perform PSG and ET co 2 monitoring.
The exclusion criteria were if he or she had chronic obstructive pulmonary disease and other cardiopulmonary disease.
2.2
Measurements
Polysomnography was performed recording electroencephalography (C3/A2, C4/A1), electro-oculogram, submental electromyography, right and left anterior tibialis surface electromyography, electrocardiogram, nasal and oral air flow, thoracic and abdominal movements, and oxyhemoglobin saturation.
End-tidal CO 2 was sampled through a Smart CapnoLine Oral Nasal Cannula (Oral-Nasal FilterLine, Nellcor, Plesanton, CA) and measured using microstream capnometer (NBP-75⁎’, Nellcor Puritan Bennett, Plesanton, CA). Capnometer was calibrated according to the manufacturer’s instructions and integrated into the standard polysomnographic recordings. All variables were recorded continuously and transferred to a computer for analysis.
2.3
Data processing
Sleep stages were scored according to standard criteria . Apnea was defined as the absence of airflow for at least 10 seconds. Hypopnea was defined as a decrease in airflow of at least 30% for at least 10 seconds with either an arousal or an oxygen desaturation of 3%. The AHI was defined as the average number of apneas and hypopneas per hour of sleep. Arousals were defined as recommended by the American Sleep Disorders Association Task Force criteria .
The ET co 2 numeric values and waveform were displayed in real time on the PSG epoch. We used the maximum amplitude of each waveform as an ET co 2 value of each breath that was calculated using our own computer algorithm (ET co 2 module, Sandman Elite Version 8.0 Sleep Diagnostic Software). Mean and maximum ET co 2 values were computer generated following the manual elimination of bad data by the sleep technicians on an epoch-by-epoch basis during scoring and were based on total recording time and total sleep time (TST) and different sleep stage.
2.4
Statistics
All numbers are expressed as arithmetic mean ± SD. Statistical calculations were performed with the aid of SPSS (version 16.0; SPSS Inc, Chicago, IL). Correlation coefficients were calculated with the Spearman’s rank correlation coefficient. Comparison of 2 groups was performed using independent-sample t test. All P values were 2-tailed, and a P value of less than .05 was considered statistically significant.
2
Materials and methods
2.1
Patients
The protocol was approved by the Institutional Review Boards of Beijing Tongren Hospital (China). All participants gave informed written consent. Thirty-eight patients who were referred for suspected OSA due to the presence of symptoms suggestive of OSA, that is, habitual snoring, daytime sleepiness, and witnessed apnea, were conducted to perform PSG and ET co 2 monitoring.
The exclusion criteria were if he or she had chronic obstructive pulmonary disease and other cardiopulmonary disease.
2.2
Measurements
Polysomnography was performed recording electroencephalography (C3/A2, C4/A1), electro-oculogram, submental electromyography, right and left anterior tibialis surface electromyography, electrocardiogram, nasal and oral air flow, thoracic and abdominal movements, and oxyhemoglobin saturation.
End-tidal CO 2 was sampled through a Smart CapnoLine Oral Nasal Cannula (Oral-Nasal FilterLine, Nellcor, Plesanton, CA) and measured using microstream capnometer (NBP-75⁎’, Nellcor Puritan Bennett, Plesanton, CA). Capnometer was calibrated according to the manufacturer’s instructions and integrated into the standard polysomnographic recordings. All variables were recorded continuously and transferred to a computer for analysis.
2.3
Data processing
Sleep stages were scored according to standard criteria . Apnea was defined as the absence of airflow for at least 10 seconds. Hypopnea was defined as a decrease in airflow of at least 30% for at least 10 seconds with either an arousal or an oxygen desaturation of 3%. The AHI was defined as the average number of apneas and hypopneas per hour of sleep. Arousals were defined as recommended by the American Sleep Disorders Association Task Force criteria .
The ET co 2 numeric values and waveform were displayed in real time on the PSG epoch. We used the maximum amplitude of each waveform as an ET co 2 value of each breath that was calculated using our own computer algorithm (ET co 2 module, Sandman Elite Version 8.0 Sleep Diagnostic Software). Mean and maximum ET co 2 values were computer generated following the manual elimination of bad data by the sleep technicians on an epoch-by-epoch basis during scoring and were based on total recording time and total sleep time (TST) and different sleep stage.
2.4
Statistics
All numbers are expressed as arithmetic mean ± SD. Statistical calculations were performed with the aid of SPSS (version 16.0; SPSS Inc, Chicago, IL). Correlation coefficients were calculated with the Spearman’s rank correlation coefficient. Comparison of 2 groups was performed using independent-sample t test. All P values were 2-tailed, and a P value of less than .05 was considered statistically significant.
3
Results
Data from 38 patients (AHI ranged from 6.4 to 92.6) were collected for analysis. The ages of patients in this study ranged from 18 to 68 years. There were 32 male patients and 6 female patients. Overall data for the subjects are displayed in Table 1 . Of all 38 patients, the wake time mean end-tidal pressure of carbon dioxide (wake ET co 2 ) was 38.77 ± 3.09 mm Hg, nonrapid eye movement mean ET co 2 was 37.78 ± 4.91 mm Hg. Rapid eye movement mean ET co 2 was 36.08 ± 6.67 mm Hg, TST mean ET co 2 was 37.43 ± 5.12 mm Hg, mean ET co 2 of difference between mean TST ET co 2 and wake ET co 2 (T − W ET co 2 ) was −1.07 ± 2.66 mm Hg (−7.8 ± 3.5 mm Hg).
